1. Field of the Invention
The present invention relates to a rotation angle detector mainly used for detecting a rotation angle of a steering wheel of an automobile.
2. Description of the Related Art
In recent years, more and more automobiles have been equipped with increasingly sophisticated equipment such as various kinds of rotation angle detectors, which detect a rotation angle of a steering wheel, for use with various controls. One of such conventional rotation angle detectors is described as follows with reference to FIGS. 5, 6A, and 6B.
FIG. 5 is an exploded perspective view of the conventional rotation angle detector. Rotator 1 has spur gear 1A on the outer periphery of the side surface thereof. Rotator 1 is provided at its center with a hole for passing a shaft of a steering wheel (unillustrated) therethrough, and on its inner periphery with engaging portion 1B to be engaged with the shaft of the steering wheel. First detecting body 2 has spur gear 2A on the outer periphery of the side surface thereof. Second detecting body 3 has spur gear 3A on the outer periphery of the side surface thereof. Spur gear 2A has a different number of teeth than spur gear 3A, and is engaged with both spur gears 1A and 3A.
Wiring board 4 is placed above first and second detecting bodies 2 and 3, nearly parallel thereto. Wiring board 4 is provided on both sides thereof with a plurality of wiring patterns (unillustrated), and mounted with magnetic detection elements 5B and 6B. Magnetic detection element 5B is placed at a position facing magnet 5A at the center of first detecting body 2, and magnetic detection element 6B is placed at a position facing magnet 6A at the center of second detecting body 3. Magnet 5A and magnetic detection element 5B together form a first detector. Magnet 6A and magnetic detection element 6B together form a second detector. On wiring board 4, controller 7 is also mounted. Controller 7 is made of an electronic component such as a microcomputer, which is connected to magnetic detection elements 5B and 6B.
The rotation angle detector is housed in case 8, which is box-shaped and made of an insulating resin. Rotator 1 has cylindrical portion 1C on its bottom surface, and case 8 has cylindrical portion 8A whose inner periphery rotatably holds cylindrical portion 1C. Case 8 further has shafts 8B and 8C. Shaft 8B rotatably holds the center of first detecting body 2, and shaft 8C rotatably holds the center of second detecting body 3.
In the rotation angle detector thus structured, controller 7 is connected to an electronic circuit (unillustrated) in an automobile via, for example, a connector or a lead wire (unillustrated). The shaft of the steering wheel passing through the hole at the center of rotator 1 is engaged with engaging portion 1B, so that the rotation angle detector is attached to the automobile. Such a rotation angle detector is disclosed in Japanese Patent Unexamined Publication No. 2006-258625, for example.
In the above-described conventional structure, when the steering wheel rotates, rotator 1 rotates and causes first detecting body 2 to rotate in association therewith. The rotation of first detecting body 2 causes second detecting body 3 to rotate in association therewith. Accordingly, magnets 5A and 6A attached at their centers also rotate. Magnetic detection elements 5B and 6B detect changes in the magnetism of magnets 5A and 6A, respectively, and output them as detection signals. Since spur gears 2A and 3A have different numbers of teeth than each other, first and second detecting bodies 2 and 3 rotate at different speeds than each other. As a result, data waveforms from magnetic detection elements 5B and 6B have different cycle lengths than each other and, therefore, become detection signals different in phase.
Controller 7 performs a predetermined calculation based on the two different detection signals obtained from magnetic detection elements 5B and 6B and on the numbers of teeth of spur gears 2A and 3A, thereby detecting a rotation angle of rotator 1. This rotation angle corresponds to a rotation angle of the steering wheel. The result is output to the electronic circuit of the automobile so as to perform various functions for the automobile.
In the rotation angle detector described above, as shown in the schematic plan view of FIG. 6A, there is a gap providing a certain clearance between the outer periphery of cylindrical portion 1C of rotator 1 and the inner periphery of cylindrical portion 8A of case 8 so that rotator 1 can rotate. The gap is only 0.1 mm or less.
When the steering wheel starts to rotate leftward (i.e., counter-clockwise), for example, rotator 1 starts to rotate via the shaft of the steering wheel and engaging portion 1B engaged therewith, thereby moving leftward an amount corresponding to the gap, as shown in FIG. 6B. Consequently, the rotation of rotator 1 is transmitted to first detecting body 2 after the left-side of the outer periphery of cylindrical portion 1C comes into contact with the left-side of the inner periphery of cylindrical portion 8A.
More specifically, immediately after the steering wheel starts to rotate, rotator 1 rotates an amount corresponding to the angle from point C to point C1. However, point D, at which rotator 1 is engaged with first detecting body 2, does not change. In other words, first detecting body 2 starts to rotate when rotator 1 has rotated from point C to point C1. Therefore, after rotator 1 starts to rotate, first detecting body 2 starts to rotate after a time delay corresponding to the gap between cylindrical portions 1C and 8A, although the time delay is very small. As a result, the conventional rotation angle detector causes slight errors in detecting the rotation angle.